The Off-Grid Home
Eco expert Tim Pullen explains what to consider when building a home away from mains services
Images: sImon maxwell
Awalk, perhaps some half a mile from the road along a track through a pretty bluebell wood, leads to a big clearing. A gentle, south-facing slope meanders down to a lake where there are ducks gliding across its shimmering top, moorhens fossicking around the margins and fish frolicking beneath the surface. In this case, it is the fish that will make a business venture possible and that will enable planning consent to be granted for a new house.
A dream scenario made real. But, there’s a problem — there’s no possibility of access to any mains services. If this house is to be built, it has to be built entirely off-grid.
This situation does come up, albeit rarely. Planning consent obtained in a remote location, that enables a house to be built to support a rural business venture. There are other self-builders that build in less remote locations but simply choose to be off-grid. In either case the challenges remain the same: the development has to produce its own heat and power, deal with its own waste and provide its own water. And, finally, there are many in rural locations who may be off-grid for some provisions (i.e. mains gas and mains drainage), but not all (say, electricity and water); the self-builder or renovator will still need to put some consideration into how these provisions will be made.
it all starts with design
Mark Waghorn, owner and director of Mark Waghorn Design, is an architect specialising in lowimpact, sustainable house design. “Keep it simple,” he begins. “The traditional house form (i.e. akin to what a child would draw) has a lot going for it. It is easy to build and has a low surface area to volume ratio, which helps with heat retention.”
The wisdom of these words can be seen in his innovatory, award-winning designs which are low-impact, with low energy consumption and are relatively cheap to build. All of which are key issues for the off-grid house.
By far the largest energy use in a typical house is in space heating, but it does not need to be that way. As Mark says: “If you are off-grid, there is an even greater imperative to minimise your heating requirement. If you build to the Passivhaus standard, for instance, the need for any space heating will be next to zero.”
But this assumes that there is some difficulty in generating heat energy. An off-grid house will be reliant on the site’s resources for energy, water and waste management, as a minimum. I know of an off-grid home in mid-Wales, set in the middle of a
70-acre woodland where the owners live and work on their land. Their house is not particularly energy efficient, but they do have access to more wood than they can ever consume in heating the house.
Considering your lifestyle before you plan is particularly key to the off-grid home. Think about the appliances you use and your daily routine and factor this into your design to keep consumption low. “Think about the type and arrangement of ancillary, external accommodation,” adds Mark. “Glazed spaces attached to the house can be useful for a multitude of different functions — drying clothes, growing food or seasonal expansion of living accommodation.” In a low-energy, off-grid home, ancillary space can also provide bootrooms, larders and biomass stores at a very low build cost, with almost zero energy demand.
There are two issues to consider: minimising the energy demand in the first instance and knowing exactly what that demand will be, as it changes day-to-day and season-to-season. Off-grid means exactly that; it is not on the grid and there is no back-up. It means that the site has to produce all it needs but, ideally not more than is needed as the surplus will just be wasted (unless there is provision for energy storage, which we’ll come back to later).
The solution starts with the most accurate possible calculation of the energy demands, divided into space heating, hot water and electricity. Industry standards suggest roughly 1,000kWh/ yr per person living in the house for hot water and 1,500kWh/yr per person for electricity. The space heating demand will vary with the size and construction of the house but Building Regulations set out a maximum permitted of 55kWh/m2/yr.
Space heating and hot water (and cooking) could be produced using bottled gas. It is inefficient, expensive and some distance from sustainable, but it is flexible and predictable. This could also be achieved with biomass (harvested or bought-in log pellets) which would be cheaper, more efficient and sustainable. Alternatively, it might be that heating is to be produced using electricity —➤probably via a
“An off-grid house will be reliant on the site’s resources for energy, water and waste management”
heat pump. That means generating more electricity and investing more in generation and storage technology.
There are two truths that have to be recognised:
l Hitting exactly 100% of demand is practically impossible. What is wanted and what is being produced will both vary with the circumstances on any given day and as a result there will always be a surplus or a shortfall.
l There is no electricity-generating technology that gives predictable production, day-to-day and season-to-season. We can predict with some accuracy what any technology will produce over the course of a year. But, as the technologies currently available at the domestic level are reliant on the weather, we can’t predict what it will produce on any given day.
What this leads to is the need for energy storage and, probably, another technology for back-up. Although, not always — if the heat energy is, for example, produced from biomass then storage is a non-issue. Electricity storage, however, means a battery bank, which leads to another calculation — how big does the battery bank need to be? If a photovoltaic (PV) solar system is the principal means of generating electricity, it’s worth knowing that the industry sets out that broadly 50% of the annual production will be in the three summer months and only 10% in the three winter months. So, do you want to store that summer surplus until winter – that will need a prodigiously big battery bank – or have a second technology, wind, say, that works better in winter?
This leads to yet another calculation. As an example: one day could be grey and overcast with no wind. Another day could see bright sunshine with plenty of wind. Easy-peasy to store a fairweather, sunny day’s surplus to keep you going on greyer days. But how long will grey conditions go on for and how big does the battery have to be?
A 5kW wind turbine will produce around 9,000kWh and a 10kW PV system will produce about 8,500kWh per year. A 1kW hydro-power scheme will produce around 8,000kWh per year. It will still be affected by the weather (more rain equals more production) but is far more reliable, and predictable, than other technologies. All of these systems produce more than enough to satisfy the demands of the house. But all will produce a surplus at some time of the year and a shortfall at others. A combination of technologies – say a 5kW PV with a 3kW wind turbine and a small battery bank – is really the only way to be sure.
“All technologies currently available at the domestic level are reliant on the weather so we can’t predict what it will produce on any given day”